The present invention relates to an image forming apparatus that appropriately controls the heater that is used as the heat source employed for fixing during image formation.
In general, in an image forming apparatus based on the xerographic method, in order to carry out thermal fixing of the toner image that has been transferred on to the entire surface of the image supporting body such as-transfer paper on to said image supporting body, very often a fixing apparatus is used that consists of a fixing heated roller in contact with the surface of said image supporting body and a pressure roller that is placed so as to be pressed against this fixing heated roller.
Further, in this type of fixing apparatus, as a heating source of the fixing heated roller, a heater such as a halogen lamp is provided inside the roller as its heat source.
In addition, a temperature sensor is provided for this heat source, and the result of detection by the temperature sensor is supplied to the single CPU used as a controlling means. Further, the single CPU will be carrying out “heat source temperature control” and “temperature abnormality detection control”.
Here, “heat source temperature control” is carrying out the control of the temperature of the heat source so that it is maintained near the set temperature by repeatedly switching on and off the power supplied to the heat source. On the other hand, the “temperature abnormality detection control” is carrying out the control of shutting off the power supply to the heat source when the temperature of the heat source goes outside a set range relative to the set temperature thereby judging that said heat source temperature control is not being carried out normally.
Further, in the temperature abnormality detection control described above, it is necessary to immediately shut off the power supply to the heat source. Therefore, if the heat source temperature control and the temperature abnormality detection control are being carried out by a single CPU, some situations may arise when it is not possible to carry out control in a normal manner of the temperature abnormality detection control.
Therefore, as a safety measure when the control could not be carried out normally, the conventional image forming apparatuses were very often provided with a separate hardware circuit that could execute the temperature abnormality detection control (temperature abnormality detection and shutting off the power supply to the heat source) without depending on semiconductors such as the CPU, etc.
By providing separately such a hardware circuit, although the reliability of temperature control of the image fixing apparatus could be maintained, there were problems created such as cost increase and more complex configuration of the image forming apparatus.
In addition, as another problem, if the different types of controls related to the entire image forming apparatus and the temperature control of the heat source are executed by a single CPU, the work load on the CPU becomes large at the time of switching on the power to the apparatus due to the tasks of initializing various sections of the apparatus thereby causing considerable time to be taken until the power supply to the heat source is started, thus causing the problem of requiring long warm-up time of the image fixing apparatus.
The present invention aims to solve problems such as the above, and the purpose of the present invention is to realize an image forming apparatus in which it is possible to carry out control of the temperature of the heat source appropriately without having to use a dedicated hardware circuit for shutting off the power supply when a temperature abnormality occurs in the heat source.